1. Field of the Invention
This invention relates to bladder assemblies for monitoring lung pressure and more particularly to a bladder assembly for matching the lung pressure of a subject with the pressure in a positive pressure breathing system.
2. Description of the Related Art
Modern high performance fighter aircraft are capable of flight maneuvers which subject the crew members to very high accelerations, "G's", particularly in the head-to-foot direction. Severe stresses are placed upon the crew members by such aircraft maneuvers. Indeed, many modern aircraft are capable of aerial maneuvers which are beyond the tolerance of the human crew members. Consequently, fighter aircraft and their crew members are occasionally lost as a result of G-induced Loss of Consciousness (G-LOC).
Many military jet aircraft have service ceilings above 40,000 feet and are equipped with oxygen administered to the air crew member through an oxygen mask worn about the mouth and nose. Typically, the oxygen system is a demand type apparatus wherein the oxygen is inhaled and exhaled as the air crew member's lungs expand and contract. However, as the aircraft cockpit reaches an altitude of 40,000 feet or more, the oxygen must be supplied to the lungs at a pressure exceeding that of the cockpit to prevent hypoxia. Typically, the aircraft will include a positive pressure breathing system which automatically supplies oxygen under pressure to the mask and into the air crew member's lungs to ensure an adequate supply of oxygen is maintained therein.
It has been found that high altitude positive pressure breathing is aided by exerting pressure against the upper portion of the body, particularly around the chest and abdominal areas, because the pressure exerted assists the air crew member in exhaling breath gases from the lungs against the positive pressure in the face mask. Therefore, an inflatable garment called a counterpressure garment is worn around the chest and back area so that during positive pressure breathing the counterpressure garment is also inflated to the same pressure as in the mask. It is also desirable in a high altitude environment to pressurize the anti-g suit to a predetermined ratio of the positive pressure in the breathing mask; e.g. typically four times mask pressure.
It has also been found that G protection is enhanced by positive pressure breathing because the increase in breathing pressure causes an approximately equal increase in heart level blood pressure, thereby increasing the flow of blood to the brain. Therefore, it is sometimes desirable to initiate positive pressure breathing at some minimum G force level. When the predetermined level of G force is encountered, or when the pressure in the anti G suit reaches an equivalent value, a signal is transmitted to a breathing regulator to initiate positive pressure breathing to the face mask, which by means of a valving system automatically initiates inflation of the counter pressure garment.
U.S. Pat. No. 4,638,791, entitled "Apparatus and Methods for Providing Rapid Protection from Accelerative Forces Experienced by Aircraft Crew Members," issued to S. B. Krogh et al., discloses regulation of pressure in a positive pressure breathing (PPB) system by movement of an aircraft control stick. This movement signals an impending aircraft acceleration. PPB and G suit levels are then adjusted to protect the pilot against this acceleration. In the Krogh disclosure, readings from airplane accelerometers are also used to control the level of PPB and G suit pressure.
U.S. Pat. No. 4,243,024, entitled "G-Protection System Sensing a Change in Acceleration and Tilt Angle", issued to R. J. Crosbie et al., discloses the use of measurements of aircraft acceleration and seat angle to control suit pressure.
U.S. Pat. No. 4,858,606, entitled "Low Pressure Breathing Regulators and Breathing Gas Systems Incorporating the Same", issued to H. A. S. Hamlin, discloses the control of the flow of air through a PPB regulator via a G sensitive valve.
All of the above mentioned inventions use indirect measures of pilot G force to regulate pressure. None of these inventions use direct measurements of the G forces on the pilots that control PPB and G suit pressure. Furthermore, none of these inventions account for body positioning. The position of the pilot's body in relation to the G force can significantly affect a pilot's G tolerance. Use of indirect measures of pilot G forces that do not account for body positioning may result in excessive or insufficient pressurization of a PPB system or a G suit.
Present assignee, Rockwell International Corporation, has previously proposed two solutions for solving the problem of high "G" accelerations. However, neither of these previously proposed solutions address the problem of body positioning and appropriate levels of pressurization for a PPB System or an anti-G suit. For example, U.S. Pat. No. 4,925,133. issued to S. G. Wurst et al. entitled "Hydraulic Buoyancy Force Suit" (assigned to Rockwell International Corporation) discloses a buoyancy force suit having at least two layers of flexible material with a substantially compressible fluid disposed therebetween. A fluid reservoir in fluid communication with the space between the layers maintains a constant fluid level with the buoyancy suit during acceleration. The fluid reservoir is supported at substantially the subject's eye level for maintaining an optimal fluid pressure gradient for ensuring an efficient blood supply to the subject's brain.
The force suit provides a balanced counterpressure to the force of acceleration, the force suit exerting pressure on substantially all body surfaces underneath the suit. However, the buoyancy force suit still requires positive pressure breathing to counter the force of the water in the suit against the chest.
U.S. Pat. No. 4,923,147, issued to H. J. Adams et al., entitled "Head Support/Spine Offloading Ejection Seat Insert" (also assigned to Rockwell International Corporation), discloses a seat insert (HS/SOD) which is insertable within the existing seat of a vehicle for maintaining a crew member of the vehicle in a forward posture during high G accelerations.
The HS/SOD works effectively to offload the spine of a pilot. However, the straps connecting the chestplate and backplate components of the seat insert must be tight. This may be problematic, there being a fine line between the tightness required for effectively offloading the spine and the loose fit required for comfort and unimpaired respiration. It is difficult to have these straps tight and yet comfortable unless the pilot's chest dimensions closely match those of the chestplate component. The problem of tightness and comfort can be alleviated by the bladder system described in this disclosure.